Session L15: Electronic Structure:Thermodynamic & Optical Properties

Infrared magnetospectroscopy study of InAs/GaSb quantum wells from normal to inverted region

Inverted InAs/GaSb quantum well bilayers have been theoretically predicted and experimentally confirmed as a quantum spin Hall insulator (a two-dimensional topological insulator) when the conduction band of InAs anti-crosses the valence band of GaSb. In this work, we study the cyclotron resonance in InAs/GaSb quantum wells in Faraday configuration and in magnetic fields up to 17 T. By changing the relative thickness of each quantum well layer, we are able to band-engineer the material from the normal region to the inverted region (quantum spin Hall region) through a phase transition (critical region). We show that in the normal and critical region only single cyclotron resonance can be observed, while in the inverted region multiple cyclotron resonances, as well as anti-crossing behavior, occur. We compare our experimental results with predictions of eight-band k$\cdot$p calculation.   

Ellipsometric Study of NbO$_{2}$ Grown by MBE on LSAT from 77 to 800 K

NbO$_{2}$ is a transition metal oxide that has been of interest for several decades. Like other complex oxides it has a metal-insulator transition provoked by external stimuli such as temperature, pressure, and electric fields. Our study shows the dielectric function of NbO$_{2}$ grown by molecular beam epitaxy, optical axis in-plane, on (LaAlO$_{3})_{0.3}$ (Sr$_{2}$AlTaO$_{6})_{0.35}$ (LSAT) substrates. The ellipsometric angles were measured from 0.76 to 6.52 eV using a UV/VIS variable-angle spectroscopic ellipsometer and from 250 to 1200 cm$^{-1}$ using an FTIR ellipsometer. By using regression analysis we modeled our optical spectra with one model over the entire range from the mid-infrared to the near UV. For the LSAT substrate, we used optical constants from a previous study. A sum of Tauc-Lorentz oscillators describes the dielectric function of the NbO$_{2}$ film. We have measured the dielectric function of the sample from 77-800 K. This has allowed us to see that the absorption peaks sharpen/broaden with decreasing/increasing temperature. We have also plotted the direct and indirect band gaps as a function of temperature.   

Lyddane-Sachs-Teller Analysis of Electronic Transitions

We have explored the use of the Lyddane-Sachs-Teller (LST) relation for analysis of electronic optical spectra. This relation originated in the theory of IR lattice absorption and, in analogy with the high IR reflectivity of polar crystals, we demonstrate a substantial region of almost-metallic UV reflectivity above the fundamental electronic absorption in selected solids; it is especially pronounced in group IV elements. This electronic \textit{Reststrahlen} is terminated by a well-defined longitudinal plasmon and may be understood within LST theory. The original LST formulation neglects dissipation; we show it reflects the geometric symmetry of a normalized dispersion curve based on relative frequencies and polarizabilities. If dissipation is included, absorption widths enter only in second order, so the original LST relation applies to optical spectra that can be approximated by the Lorentz model. The Kramers-Kronig based moments formulation of Noh and Sievers [1] holds generally. The normalized curve is specified by a single strength parameter that may be used as an approximate index to characterize optical response. The dielectric response of covalent semiconductors will be discussed as examples of the LST relation and the dependence of energy-loss spectra on electronic properties. [1] T. W. Noh and A. J. Sievers, Phys. Rev. Lett. \textbf{63}, 1800 (1989).   

Correlation between heavy-hole and light-hole Mahan Excitons in a two-dimensional electron gas

We present the coherent two-dimensional Fourier transform (2DFT) spectra of Mahan Excitons associated with the heavy-hole and light-hole resonances observed in a modulation doped GaAs/AlGaAs single quantum well. These resonances are observed to be strongly coupled through many-body interactions. The 2DFT spectra were measured using co-linear, cross-linear, and co-circular polarizations and reveal striking differences. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.   

Optical Properties of LaVO$_3$ and YTiO$_3$

Motivated by recent experimental efforts to fabricate p-n junctions from transition metal oxides (TMOs) and recent theoretical and computational work illustrating that strongly correlated insulators can produce more than one electron-hole pair from a single solar photon through impact ionization, we study the optical properties of LaVO$_3$ and YTiO$_3$ by {\it ab initio} methods. We apply the Bethe-Salpeter equations (BSE) to calculate the optical properties, starting from quasi-particle energy levels and states found at the level of the GW approximation. We present comparisons of the exciton binding energy to experimental data. We examine the extent to which these materials might be promising for efficient carrier multiplication, as has been shown previously in VO$_2$.   

FT-IR and Raman Spectroscopic Study of Cobalt Oxides

Fourier transform infrared (FTIR) and Raman spectroscopy are studied on cobalt monoxide (CoO) and cobalt dicobalt oxide (Co3O4) in the presented work. We can experimentally detect the transverse and longitudinal optical modes of Co3O4 using transmittance and diffuse reflectance (DRIFTS) measurements in FTIR, which showed good agreement with theoretical calculation. DRIFTS results also proved that Co3O4 with smaller particle size will lead to an increase in the LO-TO ratio. During the oxidation process from CoO to Co3O4, this ratio is gradually raised. CoO can be identified with a broad band near 500cm-1. For the first time, we clearly demonstrate that CoO exhibits only the second order Raman scattering near 1070cm-1 at room temperature and ambient pressure (excited by 473nm laser).   

Infrared magneto-spectroscopy of narrow bandgap metamorphic InAs$_{\mathrm{1-x}}$Sb$_{\mathrm{x}}$

Metamorphic InAsSb alloys are a promising narrow gap semiconductor with bandgaps as low as 0.12eV, corresponding to approximately 10$\mu$m, much smaller either InAs or InSb. The effect of bowing on the reduction in the bandgap over its binary counterparts is not completely understood. Here, we report on the systematic low-temperature infrared magneto- absorption study of a series of InAs$_{\mathrm{x-1}}$Sb$_{\mathrm{x}}$ layers with varying Sb concentration performed in magnetic fields up to 17.5 T. Several cyclotron resonance series, including interband transitions, are observed. These results allow us to extract and compare the bowing coefficients for the effective mass and bandgap.   

Magnetic Field-Induced Direct-Indirect Bandgap Crossover in Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As

Determining the exact alloy composition and energy at which a direct-indirect bandgap crossover occurs in semiconductor alloys is important for engineering optoelectronic materials. However, some amount of error is usually introduced when establishing the crossover from the extrapolation of the direct and indirect bandgap energy trends measured with a discrete set of alloyed samples. We use high magnetic fields up to 58 T to induce the crossover in a single Al$_{\mathrm{0.376}}$Ga$_{\mathrm{0.624}}$As sample, thereby allowing us to precisely identify the crossover energy at low temperature [1]. The onset of the crossover is marked by a reduction in the photoluminescence peak energy and the emergence of an asymmetric peak lineshape resulting from the competition between the radiative lifetime and carrier migration times in the indirect regime. Analysis of the lineshape progression through the crossover confirms that the crossover can be classified as an alloy disorder broadened, first order phase transition. [1] K. Alberi, et al, Appl. Phys. Express, 7, 111201 (2014)   

Transient Infrared Studies of Carrier Injection Effects on the Reststrahlen Band of SiC

Sub-diffraction light confinement has led to advances in imaging, metamaterials, and plasmonics among other fields. A phenomenon that can provide sub-diffraction light is the surface phonon polariton (SPhP). SPhPs couple infrared photons with optical phonons. Because SPhPs are coupled directly to phonons, lifetimes can be longer than that of surface plasmon polaritons (SPPs) whose lifetimes are dominated by electron scattering. SiC is one material that exhibits SPhPs. SiC SPhPs are activated by photons with energies near the Reststrahlen band. In this study we investigate aspects of carrier dynamics by photo-injecting free carriers into the SiC conduction band using a pulsed 355 nm pump laser and probe the resulting dynamics near the Reststrahlen band using a tunable CO2 laser. Variable pump fluences provided free carrier densities of ~1x10$^{17}$ to 1x10$^{19}$. Probing the excited state dynamics near the Reststrahlen band revealed complex transient behavior resulting in positive and negative changes in transient reflectance depending on the photo-injection level and the probe energy. Numerical simulations were carried out to mimic the initial photo-injection level provided by the transient experiment and resulted in qualitative agreement with the experiment.   

Lifetime investigations of recombination in CdTe heterostructures using time-resolved photoluminescence

Free photocarrier lifetimes are critical parameters in semiconductors used in photovoltaics, such as cadmium telluride (CdTe). However, CdTe is historically plagued by short photocarrier lifetimes due to competing non-radiative recombination attributed to surfaces and interfaces. One consequence of this is an elusive lifetime for the bulk material. We report progress in mitigating the effects of surfaces and interfaces using a CdTe/Mg$_{\mathrm{1-x}}$Cd$_{\mathrm{x}}$Te double heterostructure grown by molecular beam epitaxy. Photocarrier lifetimes are measured using time-resolved photoluminescence (TRPL). Lifetimes as high as 240 ns are measured at room temperature with barriers having approximately x $=$ 35{\%}. The results of these studies will be presented along with a discussion of the radiative, surface, and interfacial recombination.   

Phonon dispersion relations of Sb2S3 and Bi2S3 using the supercell force-constant method

We present a lattice dynamical study on the orthorhombic antimony sulphide (Sb$_2$S$_3$) [1] and bismuth sulphide (Bi$_2$S$_3$) [2] using the supercell force-constant method. We find that the slow decay of the interatomic force constants for these compounds in the Pnma setting critically demand the use of a large supercell of $2 \times 4 \times 2$ that consists of 320 atoms. To enable a practical calculation the space group information is fully utilized where only inequivalent atoms within the primitive cell are displaced for the force calculations. The effect of Born effective charges is incorporated into the method. We compare our results with that obtained from the density-functional perturbation theory. We found that smaller supercells could lead to unphysical acoustic phonon softening and lifting of the degeneracies along high symmetry directions. Our results provide a proper guideline for the use of the supercell force-constant method: the supercell size must be carefully be tested along with other parameters such as the kinetic energy cutoff, the Brillouin zone sampling or the self-consistent convergence criteria. [1] Y.Liu, K.T.E. Chua, T.C. Sum, and C.K. Gan, PCCP 16 (2014) 345. [2] Y. Zhao, K.T.E. Chua, C.K. Gan, J. Zhang, B. Peng, Z. Peng, and Q. Xiong, Phys. Rev   

Temperature dependent band gaps of GeSiSn alloys grown on Ge buffered Si substrates

Band gap engineering of Ge by controlling tensile strain and alloying with Si and Sn has attracted great interest. Ge$_{\mathrm{1-x-y}}$Si$_{\mathrm{x}}$Sn$_{\mathrm{y}}$ ternary alloy with two compositional degrees of freedom allows decoupling lattice constant and electronic structures. Hence it is important to determine the temperature and compositional dependence of optical properties of these materials. The complex pseudodielectric functions of Ge films grown on Si(100) and GeSiSn grown on buffered Ge were measured using spectroscopic ellipsometry in the 0.76-6.6 eV energy range between 77-800 K to investigate the E$_{\mathrm{1}}$~and E$_{\mathrm{1}}+\Delta_{\mathrm{1}}$~critical point (CP) energies. CP energies and related parameters were obtained by analyzing the second-derivative d$^{\mathrm{2}}\varepsilon $/d$^{\mathrm{2}}\omega $~of the dielectric function. Our experimental results are in good agreement with the theoretically predicted E$_{\mathrm{1\thinspace }}$CP energy shift of Ge on Si due to strain generated by thermal expansivity mismatch. We will discuss the compositional dependence of the E$_{\mathrm{1}}$ and E$_{\mathrm{1}}+\Delta _{\mathrm{1}}$~CP energies of GeSiSn alloys and effects of temperature on shifting CP energies.   

Optoelectronic properties of Ta$_3$N$_5$: A joint theoretical and experimental study

A joint theoretical and experimental study of the optoelectronic properties of Ta$_3$N$_5$ was conducted by means of ab initio calculations and ellipsometry measurements [1]. Previous experimental work on Ta$_3$N$_5$ has not been conclusive regarding the direct or indirect nature of light absorption. Our work found excellent agreement between the optical spectrum computed using the Bethe-Salpeter equation and the measured one, with two prominent features occurring at 2.1 and 2.5 eV assigned to direct transitions between N and Ta states. The computed optical gap, obtained from the $G_0W_0$ direct photoemission gap, including spin-orbit coupling, electron-phonon interaction, and exciton binding energy, was found to be in excellent agreement with measurements. Our results also showed that Ta$_3$N$_5$ is a highly anisotropic material with heavy holes in several directions, suggesting low hole mobilities, consistent with low measured photocurrents in the Ta$_3$N$_5$ literature. Work is in progress to compute polaronic contributions to the hole and electron mobilities and to investigate the effect of substitutional doping on the electronic structure of Ta$_3$N$_5$.\\[4pt] [1] Juliana M. Morbec, Ieva Narkeviciute, Thomas F. Jaramillo, and Giulia Galli, Phys. Rev. B 90, 155204 (2014).   

Electrically Tunable Hot-Silicon Terahertz Attenuator

We have developed a continuously tunable, broadband terahertz attenuator with a transmission tuning range greater than 10$^{\mathrm{3}}$. Attenuation tuning is achieved electrically, by simply changing the DC voltage applied to a heating wire attached to a bulk silicon wafer, which controls its temperature between room temperature and 550 K, with the corresponding free-carrier density adjusted between 10$^{\mathrm{11}}$ cm$^{\mathrm{-3}}$ and 10$^{\mathrm{17}}$ cm$^{\mathrm{-3}}$. This `hot-silicon'-based terahertz attenuator works most effectively at 450-550 K (corresponding to a DC voltage variation of only 7 V) and completely shields terahertz radiation above 550 K in a frequency range of 0.1-2.5 THz. Both intrinsic and doped silicon wafers were tested and demonstrated to work well as a continuously tunable attenuator, but they exhibited slightly different behaviors before a dramatic transmission drop at 450-550 K: intrinsic silicon wafers showed a monotonic transmission decrease with temperature while doped wafers showed a slight increase in transmission before the drop. All behaviors can be understood quantitatively via the free-carrier Drude model taking into account thermally activated intrinsic carriers.   

Photoluminescence measurements of high Sn-content Ge$_{\mathrm{1-y}}$Sn$_{\mathrm{y}}$ and Ge$_{\mathrm{1-x-y}}$Si$_{\mathrm{x}}$Sn$_{\mathrm{y}}$ grown on Ge-buffered Si

The optical properties of newly developed, high Sn-content Ge$_{\mathrm{1-y}}$Sn$_{\mathrm{y}}$ and Ge$_{\mathrm{1-x-y}}$Si$_{\mathrm{x}}$Sn$_{\mathrm{y}}$ thin films grown on Ge-buffered Si have been characterized using temperature-dependent and laser power-dependent photoluminescence (PL) measurements. The results show two distinct PL peaks related to both the direct ($\Gamma$) and indirect (L) bandgap transitions. Furthermore, the measured separation energy between the direct and indirect bandgap related PL peaks for Ge$_{\mathrm{0.948}}$Sn$_{\mathrm{0.052}}$ sample is only about 30 meV compared to the value of 140 meV for bulk Ge. This study shows a very encouraging result toward producing Ge- and Si-based direct bandgap semiconductors, whose predicted indirect-to-direct bandgap crossover could be near 6\% Sn. Clear competition between the two transitions is also observed as a function of temperature and laser power. Overall, this work represents an extensive PL characterization of Ge$_{\mathrm{1-y}}$Sn$_{\mathrm{y}}$ and Ge$_{\mathrm{1-x-y}}$Si$_{\mathrm{x}}$Sn$_{\mathrm{y}}$ materials over a wide compositional range and should be useful for the development of next-generation optoelectronic devices.